When oil and gas wells are drilled, steel pipe is commonly used as a casing in the bore hole. This casing may hang freely in portions of the bore hole, or may be cemented in place by pumping grout between the outside of the pipe and the bore hole. In subsea completions there is a pipe casing extending from the sea floor to the platform where drilling and production equipment is located.
In some situations it may be desirable to mill away a substantial length of casing in a well. This may be desired, for example, to remove cemented casing so that a well can be redrilled. It may also be desired to remove a section of casing to permit oil or gas production at a selected elevation in a well.
For this purpose, a casing mill is used. Casing mills are typically found in the form of an elongated body that can be connected to a drill string. Pilot guide means are provided at the downhole, i.e., terminal, end of the mill to axially position and guide the mill during the cutting of the casing. A plurality of cutting blades project radially from the housing to contact the casing for milling away a length of the casing when the mill is rotated. Drilling mud is pumped down the drill string to wash the steel chips up the annulus between the drill string and casing or bore. It has been conventional to employ fragments of cemented tungsten carbide for cutting along the forward surface of each blade. Each of the blades on the mill is in the form of a steel fin dressed or coated with a layer of brazing alloy matrix containing large particles of cemented tungsten carbide. Typically these may be made by crushing and screening scrap carbide. These particles are mixed with a brazing alloy and the mixture is then applied to the steel arm by melting in an oxyacetylene flame. This leaves particles of carbide more or less randomly distributed and oriented in the matrix.
Such random orientation of the carbide particles significantly limits the efficiency of cutting. The cutting edges on the particles are randomly distributed. The total quantity of carbide available for cutting is limited by the need to have a supporting matrix of brazing alloy. There is sometimes a problem of breakage of the carbide particles so that particles are lost or cutting edges are severely blunted. Further neither weight nor torque requirements are constant over the length of the mill and are different from mill to mill. Additionally, the shape and length of cuttings created by these prior mills vary to a great degree Long, thin and elastic cuttings tend to form bird's nests in the annulus around the drill string which can, in turn, increase the torque requirements to a point where the drill string cannot be rotated or where the circulation of mud is impeded.